This invention is in the field of forming thermoplastic polymers, more particularly this invention relates to a process to form amorphous, crystallizable thermoplastic polymers.
Presently, crystallizable thermoplastic polymers are formed or shaped by heating them to their softening or melting temperature and exerting force to shape the polymeric material. The formed polymeric material crystallizes to some degree upon cooling from the temperature at which it was formed. The formed article is a crystallized thermoplastic article. Reshaping of the formed thermoplastic articles involves heating the polymer until it is sufficiently soft to be reshaped. This involves heating the polymer to a softening temperature close to or above the melt temperature of the polymer. During the heating of the polymer to the melting temperature, more crystals form making it necessary to continue to heat the polymer to the softening point before shaping can occur.
U.S. Pat. No. 4,123,473 discloses a process to form a thin sheet of an amorphous blend of polycarbonate and polyester, at temperatures from 99.degree. C. (210.degree. F.) to 138.degree. C. (280.degree. F.). It does not disclose forming the polycarbonate and polyester blend and then annealing it. Rather, the goal of this patent is to maintain the formed article amorphous so that it retains its dimensions and clarity.
For the purposes of the present invention, thermoplastic polymers can be characterized by a T.sub.ch and a T.sub.cc. The T.sub.ch is the temperature upon heating the polymer at which crystal formation is first measurable. It is an indication of the lowest temperature at which crystals form upon cooling the polymer. The T.sub.cc is the temperature upon cooling the polymer from the melt at which crystal formation is first measurable. To form such a crystallizable thermoplastic polymer, the polymer has to be heated until it is soft enough to be shaped by suitable equipment. This involves heating the polymer to above the T.sub.ch during which crystals continue to form. The formation of crystals makes the polymer stiffer and more difficult to form requiring that the polymer be heated to a high enough temperature at which the stiffening effect of the crystals is destroyed. Upon heating it to such a formable temperature, the polymer can be formed. The polymer is then cooled and upon cooling below T.sub.cc crystals once again begin to form resulting in a crystallized formed polymer article. To reform the article once again requires heating to a temperature at which the crystals again disappear.
In characterizing thermoplastic polymers there are polymers that have a very low T.sub.ch. These polymers crystallize at a very low temperature. There are polymers that have very high T.sub.cc. Once these polymers are cooled from the melt, they begin to crystallize immediately and rapidly. Further, with a minimal of heating the extent of crystallinity increases. Typical crystalline polymers include polycaprolactam, polyhexamethylene adipamide, and polybutylene terephthalate. There are thermoplastic polymers which remain amorphous or noncrystalline up to their melt temperatures. In a sense these polymers have a very low T.sub.cc or a very high T.sub.ch and slow crystallization rates. Upon cooling from the melt, crystals will not form even when cooling them to very low temperatures. Upon heating them, crystals will not form before the melt temperature is reached. One such amorphous polymer is polycarbonate.